The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with magnetic resonance angiography and will be described with particular reference thereto. It is to be appreciated that the present invention is also applicable to tracking other types of moving objects using magnetic resonance imaging and is not limited to the aforementioned applications.
In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency (RF) pulses are applied to the examination region to excite and manipulate magnetic resonance in hydrogen or other selected dipoles. Gradient magnetic fields are conventionally applied to encode information in the excited resonance.
In contrast enhanced magnetic resonance angiography or drug uptake studies, it is often desired to add a contrast agent to enhance the blood image. That is, the subject is injected with a material that enhances the blood signal. The contrast agent improves the visibility of the circulatory system or specific tissues that absorb the contrast agent in the MRI image.
Typically, after the injection of the contrast agent, there is delay time before it reaches the region of interest (ROI). Various techniques are used to track the injected agent, or bolus, to determine exactly when it region of interest, i.e., when the MRI scan of the region of interest should commence.
One method of bolus tracking is to use a test bolus. A small amount of contrast material is injected into the patient and a series of MR pilot images are gathered. The resultant images are analyzed to determine the transit time of the bolus. The MR scanner is then set to commence an imaging sequence the same transit time after injection of a larger imaging dose of the contrast agent. One drawback of this method is that it is not real time. Another drawback is that this method requires two injections, i.e. more contrast agent is injected than is necessary for the image. Another drawback of this method is that it assumes metabolic functions of the subject remain constant from the pilot scan to the imaging scan. In actuality, the bolus transit time can be affected by subject position, heart rate, blood pressure, and other variables. The test bolus method assumes all of these variables to remain constant from the test sequence to the imaging sequence.
Another MR method includes real-time k-space tracking of the bolus. The contrast agent is injected into the subject and a single line of k-space is sampled repeatedly. The intensity of this single resonance signal is monitored, peaking when the bolus has arrived in the region of interest. An imaging sequence is then initiated. This method has a much higher temporal resolution, (on the order of milliseconds) but has no visual tracking of the dynamic changes of the bolus. Thus, this method is susceptible to factors that cause significant increase in signal intensity such as movement of the patient, and the like. The operator has no way to verify that the signal peak was in fact caused by the arrival of the bolus. It is possible that premature imaging will be initiated.
In another method, the subject is fluoroscopically monitored to detect the arrival of the bolus. A series of MR fluoroscopic images are generated. However, this method has low temporal resolution. Back-to-back MR fluoroscopic images are generated at intervals on the order of a second. It is difficult to interpolate the bolus arrival time accurately from images a second apart.
The present invention provides a new and improved method and apparatus combining high temporal resolution with real time bolus tracking that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance is given. A subject containing a region of interest is disposed in an imaging region of an MRI device. A contrast agent is introduced into the subject""s bloodstream. Magnetic resonance is excited, and sampled along various trajectories through k-space that oversample at least one point, the trajectories manipulated by applying gradient pulses. The magnetic resonance signals are received, demodulated and reconstructed while an intensity of the oversampled point is monitored with respect to preselected intensity criteria.
According to another aspect of the present invention, a magnetic resonance apparatus is given. A magnet assembly generates a temporally constant main magnetic field in an examination region. Gradient coils spatially encode the main magnetic field and define sampling paths therethrough. An RF coil assembly excites resonance in dipoles of a subject in the examination region. At least on RF receiver receives magnetic resonance signals. A reconstruction processor reconstructs the resonance signals into an image representation and displays them on a human readable display. An intensity monitor monitors an oversampled point in k-space.
According to another aspect of the present invention, a method of diagnostic imaging is provided. A plurality of data lines is gathered, each data line having at least one oversampled point in common, that is, all data lines intersect at least one common point. The intensity of the oversampled point is monitored. Images are produced. A higher resolution image is produced when both the intensity monitoring and the monitoring images indicate a bolus of contrast agent has arrived in an imaging region.
One advantage of the present invention is that it provides real time visual tracking of a contrast bolus.
Another advantage of the present invention is that it provides temporally high resolution data about the concentration of the bolus.
Another advantage resides in the provision for a confirmatory image.
Another advantage of the present invention is that the data acquisition window can be centered on the region of interest temporally corresponding to the bolus arrival.
Another advantage is that the imaging sequence uses image data collected prior to the detection of the bolus peak.
Another advantage resides in reduced contrast agent dose, simplified operation, and increased throughput.
Another advantage of the present invention is that it is applicable to single echo, multiple echo, full, partial, and undersampled data collection strategies.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.